Method of vapor phase oxidation



A Feb. ll, 1936. R, RlEGLER x2,030,800

METHOD 0F VAPOR. PHASE OXIDATION Filed uarcn 28, 1929 www Maag/ci;

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4 gmm l/PPLY Ill J il 1mm-on aan@ at.' 11,4153@ Ai- 4' 2,030,800

UNITED STATES PATENT oFFlc/e v ....ezvsmgv Mn'rnon'orvaron. russa oxina'l'roNv 'NJ-femwfhfw ne; n x applicati@ Mn-enea, 192s, No; 350.524

' vows '{Gr. zcc-'itsl` 'This invention relates to an improved method takes place, this vchamber and container being of carrying on chemical reacticns in the gas' or arranged inthe best possible heat exchanging vapor state or phase, and especially.v reactions relation with one another and heat generated by which take place only in the presence'effet` cat# the chemical reaction being removed as latent I l aiytic agent, and which are hencecall'ed'catalytic heat 0f vSDOliZtiOn 0I the liquid. 5 reactions, The invention `is especially Onearrangement of apparatus for carrying on to the gas phasepartial oxidation oforganiccomthe oxidation of organic compounds is shown in pounds, such, for example, as the oxidationy of the accompanying drawing wherein the single naphthalene to phthalic anhydride, theoxidation tlgure shows some of the apparatus in elevation of benzene to maleic acid. of toluene to benzaland some in vertical section, together with the 1o dehyde and benzoic acid.- of anthracenev to -anpiping connections. thraquinone and of m thyl andethyl alcohols to In order to carry on oxidation reactions such formaldehyde andA acetaldehyde. as those above mentioned, it is necessary rst to All chemical reactions may be classed either as produce a reactive vapor mixture ofthe organic p y154 exotermic reactions, those in which .the reac- Compound andair, oxygen, ozone, or other-oxygen u tion, accompanied bythe generation of heat, or containinggas. 'I'he manner of producing this as endothermic reactions those in which the rereactive vapor mixture with which, among other action is accompanied by the absorption of heat. things. the invention is concern will be pres- In either case it is usually important to maintain ently described, after the generalV construction the temperature of the reacting gases or vapors and operation of the apparatus is touched upon. 20 relatively constant because a substantial varia- Referring to the accompanying drawing, the tion in temperature-is apt to vary the character reactive VBDOI mixture is conveyed through the of the reaction so as to produce a diierent prodpipe l by which it isvdelivered to the converter 2. uct, or so as to impair the purity of the product in which it is brought, at the proper temperawhich itis desired to obtain. Thus, for example, I ture, into contact with a catalyst 3 causing the 25 in the manufactm-eofphth'alic anhydride bythe desired oxidation to take place. 'Ihe products gas phase, partial oxidation of naphthalene, the of the reaction, still inthe vapor state, are Vled l vreaction takes place in a temperature range fromv from the @Onvelier at the bOiitOm by 8. Conduit about 350 C. to about 550 C., and if the tem- 4 to, a vapor cooler 5 and then to a condenser 6( perature of the reactingv gases or vapors is al- 'I'he converter 2 comprises a metalcylinder 30 lowed to fall substantially below this range the closed at each end by Vsuitable plates, and dioxidation is liable to be incomplete so that some vided into three compartments, namely an upunoxidized naphthalene willpass the zone ofreper compartment 1, a reaction section 8, and a action and contaminate the product. Also if the lower compartment 9. The reaction section l is 33 temperature is allowed .to exceed this figure by aA preferably built up of a relatively large num? l35 substantial amount, the oxidation may be carried ber oftubes I0,l which are preferably square in beyond the desired point. and a large part of the cross section and held in slightly 'spaced relation naphthalenewillbelost duetocomplete oxidation by a tube-sheet or by metal spacers, the tubes of the naphthalene to carbon dioxide and water. being welded to the metal spacers land to each flo' In such a reaction enormous quantities of heat other. at 'the top and bottom in slightly spaced are'generated which must be'removed from the relation to vone another or to the tube sheet to zone oi' reaction as fast as produced in order formatube-bundle or tube-nest. to prevent a rise in temperature that would carry The tube sheetor the metal which is illled `in the oxidation beyond the desired point. Ex- 'by welding at the upper and lower ends oi the othermic catalytic reactions of this kind are car-f tubes .to maintain them in spaced relation-forms 45 ried out in an apparatus known as a converter. with the tubesan upper drum H extending across Various types of converters have been used, the the top of the tube bundle. A lower drum I2 catalyst being placed upon shelves, trays, or iny is Similarly formed across the bottom of the r tubes and the heat being removedv by the cirbundle. The tube bundle Vo1: nest is secured culation of a cooling uid. A form of converter within themetal cylinder by welding the drums 50 which has proven very satisfactory in operation, Il and I 2 to itsinterior walls. The tubes Il are however, consists of a reaction chamber, or catopen at their upper and lower ends for the pasalyzer chamber, containing the' catalyst.. and a sage of thereactive vapor mixture through their container for a liquid which boils at approxinteriors which constitute the reaction or 'catlmately the temperature at which the reaction alyzer chambers Il.

when used. 'Ihe only way of avoiding this foaming was to use a vaporizer of extraordinarily large The principal object ofl the present invention is vtoprovide a method of feeding'the reactive vapor mixture to the converter by which the danger of supplying an explosive mixture to the converter is avoided. Y,

` Another object of the invention is to provide for control of vapor concentration independently of the reactive vapor volume.

Another object of the invention is to provide a ready and accurate method of controlling the ratio of oxygen-containing gas to organic compound fed to the converter.

A further object of the invention is to provide a method for operating the vaporizer Ain such a way that foaming and consequent danger of injuring the converter or'catalyst is prevented.

In carrying out the present invention, the oxygen-containing gas such as air is passed through the organic compound in the liquid state, the organic compound being heated to liquefy it, if that is necessary. The liquid organic compound is maintained at a known temperature and the oxygen-containing gas is passed through it at a known temperature and rate. In order to assist in'maintaining the temperature of the liquid organic compound, ther gas is preheated to about the same temperature as that of the organic compound. The ratio of oxygen-containing gas to organic compound in the vapor thus produced, is consequently known with pertainty. The rate and velocity at which the oxygen-containing gas is passed through the organic compound is preferably, though notnecessarlly, regulated toproduce a saturated vapor.

In accordance vfith the'present invention, instead of passing, through the liquid organic compound all of the oxygen-containing gas required in the reactive yapor mixture for the desired operation of the converter, only a portion of this `gas is' passed through it. A vapor richer in the organic compoundthan desired to employ in the converter is first made in the manner thus described, theratio of oxygen-containing gas to organic compound in this vapor being accurately known, and then this vapor is diluted with a known amount'of oxygen-containing gas to produce Va non-explosive reactive vapor mixture, ,and this mixture is then passed into the converter.

By means of this procedure, it is entirely practicable to operate the converter at .ratios quite close to and preferably'above the explosive range. This can be done with safety because of the accurate and positive control over the ratio. This, moreover, is highly important from the standpoint of economical converter operation as has already been pointed 'out` By this method a positive control over thejratio is provided instead of being subject to the whims of many variables. It is always known with exactness just what the ratio is and the ratio can be changed in the right direction with certainty in case anything goes Wrong with the operation of the converter, such for example as unoxidized organic compound passing over into the condenser. Y

It is desirable to heat the diluted gas to a point where it will not cool .the reactive vapor mixture sufficient to causecondensation of the organic compound in the mixture, and the diluted gas is preferably heated approximately to the temperature of the undiluted vapor. Such condensation can be prevented, however, by imparting heat to the reactive vapor mixture on its way to the con-` verter. l i

My invention will be better understood from the following description of carrying out the invention in connection with a particular reaction, namely, the partial oxidation of naphthalene to phthalic anhydride.

The oxygen-containing gas in this case is air,

L although other gases may be used as above mentioned. 'I'he air is supplied to an air receiver 23 through a valved pipe 24 from asuitable compressor (not shown). The air,passes from the receiver 23 through line 25, then through an air meter 26 by which the rate 4of flow of the air is measured, through a valve 21 and into the air preheater 23. Leaving preheater 28 the heated be measured by a meter (not shown) kplaced in line 36'similar to the orice meter 33, or 4by taking the difference between the reading of air meter 26and orice meter 33. The ratio of air to naphthalene in the reactive vapor mixture passing into the converter through line I is controlled by regulating valves 30 and 3|. v

The vaporizer 34 comprises a closed vessel 31 for holding the naphthalene, surrounded at the' sides and bottom by a steam jacket 38 to which steam at a denite controlled temperature or pressure is supplied by pipe 39 and from which the condensed steam is drained away through pipe 4U. A body of naphthalene 4I is maintained in vessel 31 at about the level indicated inthe drawing, the naphthalene being most conveniently introduced into vessel 31 in molten condition by means of a pipe 42. A valved pipe 43 drains the lowermost point of the naphthalene-containing vessel 31 through which the higher boiling impurities which may be present in the naphthaiene may ber removed. This permits the use of crude naphthalene, the purification thereof taking place in the vaporizer.

Airsupplied to the vaporizer 34 by means of line 32 is received by the vaporizer tube' 44, the lower end of which terminates close to the bottom of vessel 31 so that the air is discharged into the molten body of naphthalene 4I near the bottom thereof. This stream of air mixing with the molten naphthalene is broken up into numerous small streams or bubbles by means of a perforated plate45 which is placed horizontally across the vessel 31 somewhat above. the lower end of val porizer tube 44. The naphthalene vaporizes and mixes with the bubbles of air as they rise through the body of molten naphthalene, and the naphthalene vapor passing through the separating baille 46 collects in the upper part of the vaporizer and passes out at the top through pipe 35.

than the vaporizaton rate ci' the naphthalene,

the airwill be saturated with naphthalene vapor,

of the vapor and 'air mixture in the vaporizer.

which has hereinbefore been referred to simply as vapor, to distinguish it from the reactive vapor mixture produced after the mixing therewith of the dilution air, th'e amount of naphthalene in the vapor must be known. The naphthalene content of the vapor is most conveniently obtained by so running the vaporizer that the air stream is saturated at a deilnite temperature. Then by measuring the temperature, lthe amount of naphthalene in the vapor is then accurately known, and can be checked by weighing the naphthalene introduced into the vaporizer.

The rate at which the naphthalene must be vaporized is determined by the size of the converter. TheF temperature of the molten naphthalene is regulated by varying the quantity and/or temperature of the steam admitted to the 'jacket 38 and the naphthalene is maintained at such a temperature that. it preferably saturates the amount of air passing through the vaporizer,

which is measured by the orice meter 33., This amount of air is limited by the capacity of the vaporizer and by theoretical considerations of maximumdesired velocity of the air in bubbling through the molten naphthalene.

These factors determine the most desirable operating conditions for the vaporizer and the most desirable vaporizing temperature. ature at which the vaporizer is operated there forewill vary for diiferent vaporizers but with Va particular vaporizer which I have operated, I

Ahave found the m'ost desirable vaporization tem-,-

perature for naphthalene to be Vbetween about 110 and 120 C.

It is of course desirable that no unoxidized naphthalene vapor pass through the converter and therefore it is desirable that the tempera- Jture of the reactive vapor mixture be at the desired reaction temperature by the time it reaches the catalyst. The heating of the reactive 4vapor mixture to the desired reaction temperature usually takes place. very largely within the upper empty portions of the convertertubes I0 and in the upper chamber 1. A

Most of the pipe connections shown' on the drawing are preferably provided with suitable lagging and/or steam jacketing to prevent loss of heat or to prevent undesired changes in temperaturebetween different pieces of apparatus.

Pipe 35 and pipe l by which the reactive vapor mixture is conducted from the vaporl'zer to the converter-are preferably provided with a steam jacket (not shown) to maintain the temperature of the reactive vapor mixture above the dew point and prevent separation.

The pressure in air receiver 23 is' maintained sufficiently high to force the air through the body of molten naphthalene and cause a flow of the reactive vapor mixture through the converter at the highest desired velocity. In thisconnection it is to be noted that the granular mass of catalyst in each of the tubes offers resistance to the ow of the reactive vapor since this vapor in y granules.

passing through thegranules must take circuitous paths following the interstices between the The converter is provided with a safety device comprising a diaphragme" mounted in an outlet from the upper compartment l and designed to give way and release the pressure within the The temper-4 converter in case the pressure should rise to an undesired amount. Such a, diaphragm is re- 'is apt to be so great as to far exceed the normal capacity of the vaporizer and cause large amounts of liquid naphthalene to be carried up into the vaporconnection pipe I and into the converter. I'he presence' of unvaporized naphthalene in the converter in quantity is dangerous as `above explained, particularly when a large quantity of air is being delivered along with it because the naphthalene is apt to become ignited and burn with such violence as to heat the converter even to red heat. l

In order to prevent this, I place a stop check valve 48 inthe air supply line 25. This valve is so constructed that it remains open as long as the quantity of air passing through it does not exceed a predetermined amount but as soon as the volume of air exceeds this amount, the valve closes automatically. The valve may be of such construction that when it is once tripped it remains closed until reopened manually or it may be so constructed that it will reopen after the ow of air has decreased below the predeter mined value at which the valve is set to oper'- ate. With either construction, in case the explovin pipes 32 and 36.

It-was mentioned above that when' the .converter 2 has been charged with fresh catalyst the catalyst at first is very active at temperatures only slightly above the boiling point of mercury Y at atmospheric pressure. At the start of a run,

therefore, the pressure on the mercury vapor is .maintained approximately at this value, and

the reacted vapor issues from the bottom chamber 9 ofthe converter at a temperature in the neighborhoodlof 275 C. to 450 C. After the converter has been in operation for from- 3 to 6 months or more, however, the activity of the .catalyst begins to depreciate and consequently the output of the converter is reduced. f

In order to bring up the rate of production of phthalic anhydride, the pressure on the mercury vapor is increased by means of valve 20 thus increasing thetemperature of the catalyst. By increasing the temperature of 4the catalyst in this-mannen its activity is increased and theL output of the converter is brought back approximately to .the original value. This increase in temperature of the catalyst, however, also increases the temperature of the reacted vapor.

The converter is operated at this increased temperature for another period and then its operating temperature is again increased. The

.sidered to be about the maximum safe operating pressure for the converter.

The function of the vapor cooler 5 is to cool the exit gases from the converter before entering the condenser, to a temperature slightly above the dew point ofthe reacted vapors, which for phthalic anhydride is about to 132 C.

Investigation has shown that the ratio of air -to naphthalene vapor in the reactive vapor mixture delivered to the converter may be varied within' extremely wide limits. .'Ihus, for example, I have successfully used a reactive vapor mixture containing as high as 30 pounds of air to 1 pound of naphthalene vapor. Investigation has also shown, however, that there is a certain range of ratios which produces reactive vapor mixtures that are highly explosive in character and which are for this reason dangerous to pass 'I'his explosive range includes into the converter. all ratlos from about 5er 6 pounds ofj air to 1 of naphthalene to about 22 or 23 pounds of air to 1 /of naphthalene. Ratios' either below or above this range are non-explosive.

It has also been shown by investigation that 'the higherthe ratio of air to naphthalene, the purer is the phthalic anhydride product generally obtained. In other words, the purest phthalicanhydride is obtained when the reactive mixture entering the converter is composed of from 23 or more lbs. of air to 1 pound of naphthalene vapor. A phthalic anhydride product of a highly satisfactory grade is obtained by employing ratios close to the upper end of thei explosive range, that is to say, above a ratio of 22 or 23 to 1. Considering the factor of economy as well as that of purity, it is therefore dey sirable to operate at ratios which are close to the explosive range.

" As referred to above, the control of the ratio' of air to naphthalene is eilected in accordance with this invention by maintaining the vaporizer at a known temperature, by regulating valves 30 and 3| noting the readings of meters 26 and 33,

Iand by regulating air preheater 28 so that the airenters the vaporizer at a known temperature. By means of this simple control ofthe ratio, the explosive range just referred to may be very easily avoided and at the sametime the converter may be safely operated at ratios very close tothis explosive range. I,

The yconcentration of the naphthalene in the reactive vapor mixture is made entirely independent ofthe volume of mixture passing through the converter. When it is desired to make a change in the ratio it can always be predicted with certainty in which direction and vto what extent the ratio will be changed by a glvenchange in setting of valve 3|. Hence the danger of feeding to the converter a mixture which will cause an explosion resulting in serious damage is ena tirely eliminated. Moreover, the greatly reduced volume of air passed to the vaporizer prevents foaming of the naphthalene in the vaporizer and injury to the catalyst which is apt to result therefrom.

It is also an easy matter to change the velo'city of the reactive `vapor mixture in the converter independentlyof the concentration of naphthalenein this mixture, whereas by the old practice of passing the total quantity of air .through the vaporizer, whenever it was desired to change the l 5 velocity, the rate of vaporizatidn was also ncvessarily changed at the same time, thus varying the temperature as well as the ratio. With the old method also, when operating with ratios above the'exploslve limit it was substantially impossible 5 to predict how a change in the setting of thevalve would affect the ratio and it frequently happened that the ratio felltoo low causing an explosion.

I claim:-

, 1. In the vapor-phase catalytic partial oxida- 10 tion of an organic compound in which a gaseousl reaction mixture containing lthe organic compound in the vapor phase and free oxygen in a predetermined proportion is passed in contact with aheated catalyst, the improvement in the. 15

method of controlling thwe proportion of organic compound to free oxygen in the gaseous reaction mixture which comprises saturating a gas co'ntaining free oxygen with the vapor of said organic compound at a known temperature, and admix- 20 ing said saturated gas with an amountof gas containing free oxygen adapted to reduce the proportion of organic compound to free oxygen to said predetermined value.

2. The methodof forming a vapor mixture 25 containing oxygen and the vapor of a normally solid organic compound in a predetermined ratio for feeding to a converter for partially oxidizing said compound which comprises liquefying a body of said compound by'heat and maintaining it at 30 Aa known temperature, passing a gas containing free oxygen through said body of liquefied organic compound to vaporizethe same, regulating the rate of `supply of said` gas to produce a vaporA mixture approximately saturated with said vapor. 35 and admixing said approximately saturated vapor mixture with additional gas containing free oxygen, the amount 'of said additional gas Abeing adapted to produce a non-explosive vapor mixture containing oxygen and said lorganic com- 40 pound in said` predetermined ratio.

3. The method of forming a. non-explosive mix-` ture .of naphthalene vapor and'air for feeding to a converter for partially oxidizing the naph- `thalene to produce `lphthalic anhydride which 45 comprises passing a current of air through a body of molten naphthalene, regulating the rate of supply of said air to produce a vapor mixture approximately saturated with naphthalene vapor and admixing said approximately saturated vapor 50 mixture with sufficient additionalV air to produce va non-explosive mixture.

4. In the vapor-phase catalytic partial oxidation of naphthalene in which a gaseous reaction mixture containing naphthalene in the vapor 55 phase andA air in a predetermined proportion is formed for passage in contact with a heated catalyst, the improvement in the method of controlling the proportion of naphthalene to air in the gaseous reaction mixture which comprises sat- 60 urating air with naphthalene vapor at arknown temperature, and admixing said saturatedV air with an amount f of air adapted to reduce the proportion of naphthalene to air to said prede- .termined value. 65

,5. The method of forming a non-explosive mixture of naphthalenel vapor and air for feeding to a converter for partially oxidizing the naphtha- Iene in the manufacture of phthalic 'anhydride which comprises, liquefying naphthalene by 'heat 70 and maintaining it at a known temperature, heating air substantially to the same temperatura. passing said heated air through the liquid naphthalene to vaporize the same, regulating the rate at which the air is' passed through the naphtna- 75.

, \\\\a,oao,soo

lene so as to produce a vapor'mixture which\is,\ free oxygen adapted to produce a gaseous reae-l approximately saturated with naphthalene vapor, \ti9n mixture containing said organic compound and admixing said vapor mixture with additional air previously heated approximately to the temperature of said naphthalene; the amount Vof said additional air being adapted to produce a-nonjexplosive mixture.

6. In the vapor phase catalytic partial oxidation of an organic compound in which a gaseous. reaction mixture containing thev organic coxnpound in the vapor phase and free oxygen in a predetermined proportion-.is passed in contact with a heated catalyst, the improvement in the method of controllingthe proportion of organic compound to free oxygen in the gaseous reaction mixture which comprises saturating an innocuous gas with the vapor of said organic compound at a known temperature, and admixing said saturated gas -with an amount of gas containing and free oxygen in said predetermined proportion.-

'7.Inthevaporphasecatalyticpartialoxidation of naphthalene in which a gaseous reactim portionis passedincontactwithaheated catalyst, the improvement in the-method of controlling the proportion of naphthalene to i'ree oxygen in the gaseous reaction mixture which com- ,prises saturating an innocuous gas with the vapor of naphthalene at a. known temperature, and admixing said saturated gas with an amount ot gas containing free oxygen adapted to produce al gaseousreaction mixture containing naphthalene and free oxygen in said predetermined proportion.

RIEWEN RIEGLER. 

